The use of replication-competent viral vectors, such as herpes simplex virus type 1 (HSV-1) vectors, is an attractive strategy for tumor therapy, because such viruses can replicate and spread in situ, exhibiting oncolytic activity through direct cytopathic effect (Kirn, J. Clin. Invest. 105:837-839, 2000). A number of oncolytic HSV-1 vectors have been developed that have mutations in genes associated with neurovirulence and/or viral DNA synthesis, in order to restrict replication of these vectors to transformed cells and not cause disease (Martuza, J. Clin. Invest. 105:841-846, 2000).
In designing viral vectors for clinical use, it is essential that ample safeguards be employed. G207 is an oncolytic HSV-1 vector derived from wild-type HSV-1 strain F (Mineta et al., Nat. Med. 1:938-943, 1995). It has deletions in both copies of the major determinant of HSV neurovirulence, the γ34.5 gene, and an inactivating insertion of the E. coli lacZ gene in UL39, which encodes the infected-cell protein 6 (ICP6) (Mineta et al., Nat. Med. 1:938-943, 1995). ICP6 is the large subunit of ribonucleotide reductase, a key enzyme for nucleotide metabolism and viral DNA synthesis in non-dividing cells but not dividing cells (Goldstein et al., J. Virol. 62:196-205, 1988). In addition to being the major determinant of HSV neurovirulence (Chou et al., Science 250:1262-1266, 1990), ICP34.5 also functions by blocking host cell induced shutoff of protein synthesis in response to viral infection (Chou et al., Proc. Natl. Acad. Sci. U.S.A. 89:3266-3270, 1992). This is likely responsible for the less efficient growth of γ34.5− mutants compared to wild-type HSV, which has been observed in many tumor cell types (McKie et al., Br. J. Cancer 74:745-752, 1996; Andreansky et al., Cancer Res. 57:1502-1509, 1997; Chambers et al., Proc. Natl. Acad. Sci. U.S.A. 92:1411-1415, 1995). This double mutation confers important advantages: minimal chance of reverting to wild type, preferential replication in tumor cells, attenuated neurovirulence, and ganciclovir/acyclovir hypersensitivity. G207 effectively kills multiple types of tumor cells in culture and in mice harboring tumors subcutaneously or intracranially (Mineta et al., Nat. Med. 1:938-943, 1995; Yazaki et al., Cancer Res. 55:4752-4756, 1995; Toda et al., Hum. Gene Ther. 9:2177-2185, 1998; Todo et al., Hum. Gene Ther. 10:2741-2755, 1999; Chahlavi et al., Neoplasia 1:162-169, 1999; Kooby et al., FASEB J. 13:1325-1334, 1999; Lee et al., J. Gastrointest. Surg. 3:127-133, 1999). In several syngeneic tumor models in immunocompetent mice, oncolysis caused by intraneoplastic inoculation of G207 elicited a systemic immune response and tumor-specific cytotoxic T lymphocytes (Todo et al., Hum. Gene Ther. 10:2741-2755, 1999; Toda et al., Hum. Gene Ther. 10:385-393, 1999; Todo et al., Hum. Gene Ther. 10:2869-2878, 1999).
G207 has minimal toxicity when injected into the brains of HSV-1-susceptible mice or nonhuman primates (Hunter et al., J. Virol. 73:6319-6326, 1999; Sundaresan et al., J. Virol. 74:3832-3841, 2000; Todo et al., Mol. Ther. 2:588-595, 2000). Recently, G207 has been examined in patients with recurrent malignant glioma (Markert et al., Gene Ther. 7:867-874, 2000), and the results from this phase I clinical trial indicate that intracerebral inoculation of G207 is safe at doses of up to 3×109 plaque forming units (pfu), the highest dose tested. While the use of oncolytic viruses is a promising approach for cancer therapy, the therapeutic benefits will likely depend on the dose and route of administration, the extent of intratumoral viral replication, and the host immune response.
HSV-1 infection causes down-regulation of major histocompatibility complex (MHC) class I expression on the surface of infected host cells (Jennings et al., J. Virol. 56:757-766, 1985; Hill et al., J. Immunol. 152:2736-2741, 1994). The binding of ICP47 to the transporter associated with antigen presentation (TAP) blocks antigenic peptide transport in the endoplasmic reticulum and loading of MHC class I molecules (York et al., Cell 77:525-535, 1994; Hill et al., Nature 375:411-415, 1995; Früh et al., Nature 375:415-418, 1995). The binding of ICP47 is species-specific for TAPs from large mammals (Jugovic et al., J. Virol. 72:5076-5084, 1998), with the affinity for murine TAP about 100-fold less than for human (Ahn et al., EMBO J. 15:3247-3255, 1996).